1. Field of Invention
The present invention relates to a mold for gravity casting and a gravity casting method using the same. More particularly, the present invention relates to a mold for gravity casting and a gravity casting method using the same, in which a turbine housing having a twin scroll unit and an exhaust manifold are integrally cast.
2. Description of Related Art
In general, a gravity casting method is a casting method of solidifying injected molten metal within a cast (that is, a mold) using gravity of the molten metal and is characterized in that the cooling speed of the molten metal is fast and a crystal grain is fine.
FIG. 1 is a schematic view of a common gravity casting method. A mold applied to the conventional gravity casting method includes a cast 111 configured to have an upper cast and a lower cast, an injection port 115 configured to have molten metal of high temperature injected therein, a sprue 117 filled with the injected molten metal of high temperature, a runner 119 configured to transfer the molten metal filled into the sprue 117, a cavity 113 filled with the molten metal, transferred through the runner 119, and configured to cast a product, and a riser 121 connected to the runner 119 between the sprue 117 and the cavity 113, filled with the molten metal, and configured to provide the molten metal when the molten metal filled into the cavity 113 is cooled and thus the volume of the molten metal is reduced.
Accordingly, in the gravity casting method using the mold constructed as above, when the molten metal of high temperature is injected through the injection port 115 of the cast 111 using a ladle, the injected molten metal of high temperature is filled into the sprue 117.
The molten metal filled into the sprue 117 is flown through the runner 119 and filled into the riser 121 and the cavity 113. Next, after the molten metal is filled into the cavity 113, the molten metal within the sprue 117 is heated by a heating burner 123 so that the molten metal is not early clotted.
In order to prevent a crack from being generated in a cast product owing to a volume reduced when the molten metal filled into the cavity 113 is cooled, the molten metal filled into the riser 121 is supplied to the cavity 113 in order to supplement reduced molten metal.
The gravity casting method is particularly applied to the cast of an engine, including a cylinder head and a cylinder block in a vehicle maker, a camshaft, a crankshaft, an intake and exhaust manifold, and a turbine housing. Bodies for the components are fabricated and then produced into finished products through several processing processes.
Meanwhile, with the recent increasing use of a gasoline turbocharger, a method of integrally fabricating a twin scroll turbocharger and an exhaust manifold by optimizing the shape of an exhaust system is being in progress as part of improved durability and sealing and profits, as shown in FIG. 2.
That is, a turbine housing 201 for the twin scroll turbocharger is integrally cast with the exhaust manifold 203 consisting of four exhaust runner units 209. The turbine housing 201 includes a bypass unit 207 and a twin scroll unit 205, including one-side and the other-side scroll units 205a and 205b, all of which are formed of an internal space portion of the bypass unit 207.
In the conventional mold for gravity casting for integrally casting the exhaust manifold 203 and the turbine housing 201 into one body, as shown in FIG. 3, the cavity C1 of the exhaust manifold 203 and the cavity C2 of the turbine housing 201 are formed within the mold, and risers H1 and H2 respectively connected to the cavity C1 of the exhaust manifold 203 and the cavity C2 of the turbine housing 201 are also formed.
Furthermore, a plurality of gates G is formed through a gate core such that a runner R, connecting the cavity C1 of the exhaust manifold 203 and the cavity C2 of the turbine housing 201 together, is formed within the mold.
In the mold, exhaust runner cores 211, forming the respective exhaust runner units 209 of the exhaust manifold 203, are formed within the cavity C1 of the exhaust manifold 203. A twin scroll unit core and a bypass unit core, respectively forming the twin scroll unit 205 and the bypass unit 207 formed of the internal space portion of the turbine housing 201, are formed within the cavity C2 of the turbine housing 201.
Meanwhile, the twin scroll unit core has a characteristic in that the cross section thereof is reduced. Accordingly, coating is applied to a face where each scroll unit is formed in order to prevent surface adherence when cast is performed.
In order to cast the exhaust manifold 203 having the turbine housing 201 integrated therewith using the conventional mold for gravity casting, as shown in FIG. 3, molten metal of high temperature is injected through a sprue S at the top of the combined mold.
The molten metal is supplied to the underlying runner R through the sprue S and then filled into the cavity C2 of the turbine housing 201 and the cavity C1 of the exhaust manifold 203. Next, the riser H1 on the exhaust manifold side and the riser H2 on the turbine housing side are filled with the molten metal.
Here, the molten metal supplied to the cavity C2 of the turbine housing 201 is uniformly filled into the twin scroll unit 205 and the bypass unit 207 within the complex cavity C2 of the turbine housing 201 through the gates G of the gate core from the runner R. In the process of the twin scroll unit 205 being filled with the molten metal, the other-side scroll unit 205b on the lower side of FIG. 3 is first filled, and the one-side scroll unit 205a on the upper side of FIG. 3 is then filled.
Furthermore, in the state where both the cavity C2 of the turbine housing 201 and the cavity C1 of the exhaust manifold 203 within the mold are filled with the molten metal, if the molten metal is contracted in a cooling process, pressurized molten metal filled into the turbine housing-side riser H2 and the exhaust manifold-side riser H1 is supplied to supplement the contraction amount of the molten metal.
In the conventional mold for gravity casting and the conventional gravity casting method, however, the cast is formed such that the turbine housing integration type exhaust manifold is cast in a vertical direction within the cast.
In other words, in the cast, the exhaust runner units 209 of the exhaust manifold 203 are disposed up and down and then connected to the exhaust manifold-side riser H1 on the vertical side. Furthermore, the twin scroll unit 205 of the turbine housing 201 has the scroll units 205a and 205b on one side and on the other side are disposed up and down, so that the position of the turbine housing-side riser H2 corresponds to the one-side scroll units 205a on the upper side.
In characteristics in the cast direction of the conventional turbine housing integration type exhaust manifold, in the case where the turbine housing integration type exhaust manifold is cast using the conventional mold for gravity casting, when the molten metal of high temperature comes in contact with the faces where the scroll units of the twin scroll unit core having coasting applied thereto are formed, the coasting is evaporated, thereby generating gas.
The gas must be moved from a portion where the cross section of the twin scroll unit 205 is small to a portion where the cross section of the twin scroll unit 205 and exhausted. However, there are disadvantages in that the exhaust is not free because the cavity C2 of the turbine housing 201 is narrow and thus defects or a contraction hole are generated in the surface of a product because the molten metal and the gas are mixed.
That is, from a solidification analysis result of FIG. 4, it can be seen that residual stress is generated near the end portion A of the twin scroll unit 205 having the narrow cross section and thus a contraction hole is internally generated.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.
The information disclosed in this Background section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.